The present invention relates to a semiconductor sensor and, more particularly, to a field effect transistor type sensor for detecting a specific component in a solution.
A chemical sensor of field effect transistor (FET) type, i.e., ISFET (ion selective field effect transistor), comprises a silicon substrate, source and drain regions which are formed on the surface of the substrate, and an insulative film. A gate portion of the sensor is dipped in a solution so that a change in conductance between the source and drain regions, which corresponds to an ion concentration in the solution, is detected. In such a chemical sensor, since the gate portion is placed in direct contact with the solution, a contact surface thereof must be insulated. For this purpose, the device is coated with a gate insulative film (a silicon oxide film, or the like), and an insulative film acting as a protective film or a passivation film e.g., a silicon nitride film. Part of the insulative film is selectively etched to form a wiring layer connected to source and drain diffusion layers, and a metal deposition film or metal wiring is deposited on the etched portion, thus preparing a connecting portion for the external circuit.
Chemical sensors having structures shown in FIGS. 1A, 1B, 2A, 2B, and 3 have been known.
The sensor shown in FIGS. 1A and 1B has a probe-like shape. In the sensor shown in FIGS. 1A and 1B, n.sup.+ -type drain region 2 is formed on the surface of p-type silicon substrate 1, and n.sup.+ -type source region 3 is so formed on the surface of the substrate as to surround drain region 2. Silicon oxide film 4 and silicon nitride film 5, which respectively serve as a gate insulative film and a protective film, are deposited on the entire surface of silicon substrate 1. In this structure, a gate portion of the FET is defined by the source and drain regions at one side of substrate 1, a channel region therebetween, and the insulative film. Insulative film 4 on the other side of source and drain regions 2 and 3 is selectively etched, and metal films 6 and 7 serving as contact layers are formed thereon.
In the chemical sensor of this type, however, when part of silicon substrate 1 is exposed, current leakage occurs between elements in the solution. Because of this, insulative film 4 must be formed on the entire circumferential surface of substrate 1. Since wafers cannot be directly subjected to an insulative film forming process, chips having an outer shape like the sensor must be cut from the wafer, and thereafter, the insulative film must be formed on individual chips. For this reason, this conventional method is unsuitable for mass-production, and the chips are easily damaged during their manufacture. Even if a chip is not damaged during manufacture, the resultant sensor has a decrease in mechanical strength because it receives a liquid pressure on only one side of substrate 1.
A sensor shown in FIGS. 2A and 2B has an SOS (Silicon On Sapphire) structure. In this sensor, p-type silicon island layer 12 is formed on sapphire substrate 11, and n.sup.+ -type source and drain regions 13 and 14 are formed thereon. Silicon oxide film 15 and silicon nitride film 16, respectively serving as a gate insulative film and a protective film, are formed on the surface of the silicon layer 12. In the sensor of this type, a gate portion of the FET is constituted by the source and drain regions, a channel region therebetween, and the insulative film on one side of silicon layer 12. Part of the insulative film is selectively etched on the other side of source and drain regions 13 and 14, and metal films 17 and 18 are formed as contact layers on the etched portion.
In the chemical sensor of the SOS structure, all the manufacturing processes can be performed in a planar process, allowing mass-production. When a plurality of elements are formed and each has a multistructure, element isolation is complete. However, since silicon layer 12 epitaxially grown on sapphire substrate 11 is thin (e.g., 1 .mu.m or less), the wiring resistance of source and drain regions 13 and 14 becomes high, thus impairing the sensitivity of the sensor.
When the SOS substrate is used, it is necessary to dope Al from the sapphire layer. Further, the SOS substrate is inferior to bulk silicon in terms of crystallinity. The necessary Al doping and the poor crystallinity result in various problems such as low hole mobility and short lifetime. In addition, the SOS substrate is every expensive due to the use of a sapphire layer. Still further, the crystal defect will likely to increase when the substrate is heat-treated, because of the difference in thermal expansion coefficient between silicon and sapphire.
In a sensor shown in FIG. 3, n.sup.+ -type source and drain regions 22 and 23 are formed on a major surface of p-type silicon substrate 21, and insulative film 24 (e.g., a silicon nitride film or silicon oxide film) which serves as a gate insulative film and a protective film is also formed thereon. A portion between source and drain regions 22 and 23 serves as channel region 25. A gate portion of the FET is constituted by source and drain regions 22 and 23, channel region 25, and insulative film 24. Portions of insulative film 24 corresponding to source and drain regions 22 and 23 are selectively etched, and metal films 26 and 27 connected to source and drain regions 22 and 23 are deposited on the etched portions. Metal films 26 and 27 are also connected to lead wires 28 and 29. The chemical sensor with this structure is adhered to measurement tube 30, which is partially notched, by resin 31 so as to cover the connecting portions of metal films 26 and 27 and lead wires 28 and 29. The gate portion is dipped in solution 32 in tube 30 for measurement.
The chemical sensor of this structure can be manufactured by a planar process and is suitable for massproduction. However, lead wires 28 and 29 may become disconnected or peeled from metal films 26 and 27 in hardening the resin during the manufacturing process. In addition, when this sensor is used, resin 31 which is present at the same side as the detection surface is also dipped in solution 32 and expands, thus impairing its insulative property.
It would be difficult to form many sensor elements on the same substrate, without avoiding mutual interference among the elements.
In the above three types of sensors, after the passivation film is formed, it must be partially etched so as to form source and drain contact holes. Therefore, a material which is hard to be etched cannot be used as the passivation film. Although a silicon nitride film used as the passivation film can be easily etched by a reactive ion etching method or the like, it does not always have satisfactory passivation characteristics and ion selectivity. To achieve these qualities, an Al.sub.2 O.sub.3 or Ta.sub.2 O5 film is preferably used. However, since these films are hard to etch, contact portions thereof must be masked. In this case, an electron beam deposition method or a spattering method which allows a low-temperature treatment is used, but traps may be produced in an interface between the substrate and the insulative film and the resultant films have poor characteristics. Although films formed by a CVD method have good characteristics, an appropriate masking material cannot be found because of high-temperature treatment in the CVD method.